Product coating method and apparatus

ABSTRACT

A method of coating a product ( 1 ), carried on a surface of a delivery mechanism ( 3 ), with a coating substance ( 8 ). The method comprises delivering the coating substance ( 8 ) to a location ( 15 ) above said surface carrying said product ( 1 ), and allowing the coating substance ( 8 ) to fall under gravity in the direction of said surface. During its fall, the coating substance ( 8 ) is subjected to at least one pressurised gas stream, and at least one electric field, whereby the failing coating substance ( 8 ) is both dispersed and charged.

The present invention relates to a method and apparatus for the coatingof products and is applicable in particular, though not necessarily, toa method and apparatus for coating food products.

There exists a wide variety of products which require to be coated witha substance (e.g. powder, liquid, suspension, etc) during theirpreparation. For example, snack foods such as potato crisps and cornchips are usually coated with a flavouring. It is generally desirable tobe able to achieve a uniform and controllable coating over the entiresurface of a product. It is also desirable to be able to minimisewastage of the coating substance, and to reduce the dispersion of thecoating substance into the environment around the products.

A traditional method used for coating snack foods involves the use of arotating drum which has its axis generally aligned with the horizontal,but with a slight tilt. Products introduced into one end of the drumtend to travel to the other end as the drum rotates. Flavouring in theform of a powder is delivered via an inclined chute to a location withinthe drum so that the flavouring is sprinkled onto the products to becoated. This technique tends to result in a non-uniform coating ofproducts, with flavour concentration varying between product pieces andacross the surfaces of individual pieces.

It has been recognised that coating uniformity may be improved byplacing a charge on the powder grains, using for example a wireelectrode located in the transit path of the product. If the productdelivery mechanism, e.g. a rotating drum, is grounded, the products tobe coating will also tend to be grounded as they pass under theflavouring delivery mechanism. Charged flavouring grains will beattracted to the products in a generally uniform manner, in turnresulting in a generally uniform coating.

U.S. Pat. No. 5,287,801 describes apparatus for coating food products,and which comprises a screw conveyor for delivering a powder coatingsubstance to a delivery chamber. Slots are formed in the base of thedelivery chamber so that the delivered substance drops through the slotsand falls onto a product delivery conveyor. An electrostatic charginghead is located between the delivery chamber and the product deliveryconveyor, so that the coating substance falls through an electric fieldformed by the charging head, acquiring a charge as it does so.

A problem with apparatus of the type described in U.S. Pat. No.5,287,801 is that the slots in the delivery chamber can become blocked,particularly where the surrounding environment is hot and possibly humid(this is often the case in a food production facility). A furtherpotential problem is that the coating substance is not broken down intosufficiently small grains by the delivery system. Again, this problemmay be exacerbated by a warm and humid atmosphere.

An alternative electrostatic coating apparatus comprises a venturi tubethrough which the coating substance is forced by a pressurised gas. Uponemerging from an exit nozzle, the substance passes over a chargingelectrode. Again, a blockage problem can arise as the coating substancebuilding up within the venturi tube. Such blockage results in productiondowntime, increasing operating costs and decreasing productivity.

According to a first aspect of the present invention there is provided amethod of coating a product, carried on a surface of a deliverymechanism, with a coating substance, the method comprising:

-   -   delivering the coating substance to a location above said        surface carrying said product;    -   allowing the coating substance to fall under gravity in the        direction of said surface;    -   during its fall, subjecting the coating substance to at least        one pressurised gas stream, whereby the falling coating        substance is dispersed, and to an electric field, whereby the        coating substance is charged.

The present invention is applicable in particular to the coating ofsnack food products with a dry, powdered flavouring. However, theinvention may also be used to coat other products including, but notlimited to, pharmaceuticals. The invention may also be used to coatproducts with substances other than dry powdered substances including,for example, liquids and suspensions.

An advantage of the present invention is that the mechanism fordelivering the coating substance to a location above the productdelivery surface can be made relatively simple. As a pressurised gas isused to disperse the coating substance, there is no need for finedelivery slots or holes which can become easily blocked.

Preferably, the coating substance is delivered to a location above saidsurface carrying said product via an inclined chute. More preferably,the chute is vibrated or shaken both to disperse the coating substanceand to aid transfer of the substance along the chute. Preferably, anexit end of the chute has a width of 75 mm or less.

The coating substance may be delivered to said chute from a supply tankor hopper using, for example, a screw conveyor.

Preferably, the coating substance is subjected to said at least onepressurised gas stream and to said electric field immediately beneath anexit end of the chute.

According to a second aspect of the present invention there is providedapparatus for coating a product, carried on a surface of a deliverymechanism, with a coating substance, the apparatus comprising:

-   -   a conveyor for conveying the coating substance to a location        above said surface carrying said product, an exit end of the        conveyor being suitable for siting above said surface;    -   a gas jet nozzle for location substantially beneath the exit end        of the conveyor;    -   an electrode attached to or located adjacent to said nozzle;    -   a source of pressurised gas and means for coupling said source        to said gas jet nozzle; and    -   means for charging said electrode;    -   wherein in use gas ejected from said nozzle passes over said        electrode and is charged, before impinging on the coating        substance falling from the exit end of the conveyor.

Preferably, said conveyor is a chute which in use is inclined at anangle to the horizontal. The apparatus may further comprise means forvibrating or shaking the chute.

Preferably, said gas jet nozzle is attached to an underside of theconveyor, so that said nozzle is located directly beneath the exit endof the conveyor. More preferably, any of said gas jet nozzle, saidelectrode, said source of pressurised gas and said charging means aresubstantially decoupled from the remainder of said apparatus. Anadvantage of decoupling these components is that the electricalconnections therein are then not subject to the vibrations that theymight otherwise be subject to when coupled to the chute.

According to a third aspect of the present invention there is providedapparatus for use in coating a product with a coating substance, theapparatus comprising:

-   -   a gas or liquid jet nozzle having means for coupling the nozzle        to a supply of pressurised gas or liquid;    -   a nozzle holder to which the jet nozzle can be removably        attached;    -   a needle electrode attached to the nozzle holder and means for        coupling the electrode to a high voltage charging means; and    -   the needle electrode being located such that in use when gas or        liquid is ejected from the nozzle the gas or liquid passes        through an electric field created by the electrode.

A needle electrode will generate a charging field which is moreefficient at charging a gas or liquid ejected from the nozzle, thanalternative electrode designs such as a wire electrode. Preferably, theneedle electrode is relatively short, e.g. 1 mm or less.

It will be appreciated that the design of the apparatus makes it easy tochange the jet nozzle, e.g. from a nozzle designed to spray gas to anozzle designed to spray liquid.

For a better understanding of the present invention and in order to showhow the same may be carried into effect, reference will now be made tothe accompanying drawings in which:

FIG. 1 illustrates in partial cross-section apparatus for coating asnack food with a flavouring;

FIG. 2 illustrates a nozzle and nozzle mounting member of the apparatusof FIG. 1; and

FIG. 3 is a flow diagram illustrating a method of coating a snack foodwith a flavouring;

FIG. 4 illustrates in partial cross-section the apparatus of FIG. 1 withthe apparatus of FIG. 2 substantially decoupled from the main body ofthe apparatus; and

FIG. 5 illustrates in side view the apparatus of FIG. 2 with theelectrode and charging means of the apparatus substantially decoupledfrom the nozzle and nozzle mounting member of the apparatus.

There is illustrated in FIG. 1 apparatus for coating a snack food with aflavouring. For the purpose of illustration, it is assumed here that thesnack food is potato crisps. Previously baked crisps 1 are fed into anopening 2 at one end of a metal drum 3 which is mounted in a supportframe (not shown in the Figure). The drum 3 has its axis 4 orientedsubstantially horizontally, but tilted at a slight angle (e.g. 10degrees) thereto, and is rotated (by a mechanism not shown in FIG. 1) sothat the crisps travel from the end of the drum at which they enter, tothe opposite end. The crisps tumble as they travel, until eventuallythey fall out of the exit end of the drum 3 onto a moving conveyor 5which delivers the crisps to packaging apparatus (not shown). The drumis connected to ground potential.

Located on top of a support frame (not shown in FIG. 1) above the drum 3is a hopper 7 into which is loaded a dry, powdered flavouring 8. Anopening 9 in the base of the hopper 7 is coupled to a screw conveyor 10which comprises a screw 11 mounted within a cylindrical tube 6. Thescrew 11 is rotated within the cylindrical tube 6 by a suitable motor.An end of the screw conveyor 10 leads into a vertically oriented feedtube 12, the open lower end of which is located directly above aninclined chute 13.

Coupled to an underside of the chute 13 is a vibration motor 14 arrangedto vibrate the chute at a relatively high frequency but with arelatively small amplitude. The motor 14 may be for example a modelFoodTech FTO1 supplied by FMC.

The lower or exit end 15 of the chute 13 projects into the centre of thedrum 3, the exit end having a width of about 25 to 75 mm. A nozzlemounting 16 is located beneath the lower end of the chute 13, and isillustrated in more detail in FIG. 2. The nozzle mounting 16 comprises asingle moulded piece of plastics material, having a generallycylindrical passage 17 passing therethrough, close to one end of themounting 16. The passage is designed to receive a complimentary shapedand sized nozzle 18 which, once inserted into the passage 17, can besecured therein with a screw, clip, or similar fastening.

Fixed into the mounting 16 is a short needle electrode 19, located inthe passage 17 just beneath the mouth of the nozzle 18. The electrode 19is coupled via a conductor 20 to a high voltage charging circuit (forexample a cascade arrangement). In use, the electrode is charged up to avoltage of around 85 KV. An end of the nozzle 18 is coupled via a hose21 to a source of pressurised air (not shown in the figures). The nozzlemounting 16 and the various components attached thereto are designed towithstand the high ambient levels of heat and moisture, and to besuitable for use on a food production line. A suitable control unit isused to control the pressure of air supplied to the nozzle 18 and thevoltage applied to the electrode 19.

Having explained the structure of the apparatus with reference to FIGS.1 and 2, the operation of the apparatus will now be described. With thehopper 7 loaded with flavouring powder 8, the screw conveyor 10 isoperated to drive powder 8 into the top of the feed pipe 12. Powderdrops through the pipe 12 and lands on the chute 13. Due to the inclinedangle of the chute and the vibration of the chute by the motor 14, thepowder 8 is conveyed along the chute to its exit end 15. It will beappreciated that vibration of the powder within the chute will result insome dispersion of the powder, i.e. larger lumps will tend to be brokenup into smaller lumps or grains.

As the powdered flavouring 8 reaches the exit end 15 of the chute 13, itwill spill over the edge and will begin falling under gravity (asindicated by reference numeral 22 in FIG. 1). Directly beneath the exitend 15 of the chute, pressurised air is forced through the nozzle 18 andpasses through the electric field created by the charged electrode 19.The air becomes charged as it passes through this field. The charged airthen impacts on the falling powder 8, tending to disperse the powder andto break up particles to into micro sizes (i.e. 45-75 microns indiameter). The air also imparts a charge to the powder particles andgrains. It is noted that smaller particles (i.e. of micron sizes) accepta charge more easily than larger particles.

As the drum 3 is grounded, the crisps 1 travelling through the drum willalso be grounded. The falling, charged powder particles and grains willtherefore be accelerated towards the crisps 1, landing on the crisps andcoating their exposed surfaces. Due to the electrostatic forces, evenback and undersides of the crisps will tend to be coated, resulting in arelatively uniform surface coating. The method described above isfurther illustrated in the flow diagram of FIG. 3.

It will be appreciated that the apparatus and method described above donot require any small aperture components, e.g. Venturi tubes, slots, orholes, through which the flavouring powder must pass. There is thereforelittle likelihood of components becoming blocked, resulting in lessdowntime and higher productivity.

A further embodiment of the invention is illustrated in FIG. 4. Allaspects of the apparatus as illustrated in FIGS. 1 and 2 are present,the sole difference being that the nozzle mounting 16 and all componentstherein are decoupled from the remainder of the apparatus. It will beappreciated that the potentially fragile connections within the nozzlemounting, notably the means for charging the electrode 19, are then notsubject to the vibrations of the chute 13.

A further embodiment of the invention, as illustrated in FIG. 5, alsoadopts this approach. All aspects of the apparatus as illustrated inFIG. 1 are present. The electrode 19 and its charging means, however,are substantially decoupled from the remainder of the nozzle 18 andnozzle mounting member and lie in the path of the falling coatingsubstance. The electrode and its charging means are again isolated fromthe vibrations of the chute 13. The electrode is positionedsubstantially proximate to the end 15 of the chute such that asubstantial proportion of the falling coating substance is charged. Thedirection of motion of the coating substance is indicated by arrow 23.

It will be appreciated by the person of skill in the art that variousmodifications may be made to the above described embodiment withoutdeparting from the scope of the present invention. For example, wherelarger amounts of flavouring must be delivered on top of the product tobe coated, a wider chute or multiple chutes, arranged side by side, maybe used. A corresponding set of nozzles may be employed to charge anddisperse the flavouring. Alternatively, a wide mouthed knife nozzle maybe used.

1. A method of coating a product, carried on a surface of a deliverymechanism, with a coating substance, the method comprising: deliveringthe coating substance to a location above said surface carrying saidproduct via an inclined chute down which the coating substance fallsunder gravity; allowing the coating substance to fall under gravity inthe direction of said surface from the end of the inclined chute; duringits fall under gravity, and substantially immediately beneath the exitend of said inclined chute, subjecting the coating substance to at leastone pressurised gas stream, whereby the falling coating substance isdispersed, and to an electric field, whereby the coating substance ischarged.
 2. A method according to claim 1, and comprising vibrating orshaking the chute to disperse the coating substance and to aid transferof the substance along the chute.
 3. A method according to claim 1 or 2,wherein the Pressurized gas stream is subjected to said electric fieldprior to impinging upon the coating substance.
 4. A method according toclaim 1, wherein the pressurized gas stream impinges upon the coatingsubstance prior to subjection to said electric field.
 5. Apparatus forcoating a product, carried on a surface of a delivery mechanism, with acoating substance, the apparatus comprising: an inclined chute forconveying the coating substance to a location above said surfacecarrying said product, an exit end of the chute being suitable forsiting above said surface; a gas jet nozzle for location substantiallyimmediately beneath the exit end of the chute; an electrode attached toor located adjacent to said nozzle: a source of pressurized gas andmeans for coupling said source to said gas jet nozzle; and means forcharging said electrode; wherein in use gas ejected from said nozzlepasses over said electrode and is charged, and impinges on the coatingsubstance falling from the exit end of the chute.
 6. Apparatus accordingto claim 5 and comprising means for vibrating or shaking the chute. 7.Apparatus according to claim 5 or 6, wherein in use the gas ejected fromsaid nozzle passes over said electrode prior to impinging upon thecoating substance.
 8. Apparatus according to claim 5, wherein in use thegas ejected from said nozzle impinges upon the coating substance priorto passing over said electrode.
 9. Apparatus for use in coating aproduct with a coating substance, the apparatus comprising: a gas orliquid jet nozzle having means for coupling the nozzle to a supply ofpressurized gas or liquid; a nozzle holder to which the jet nozzle canbe removably attached; a needle electrode attached to the nozzle holderand means for coupling the electrode to a high voltage charging means;and the needle electrode being located such that in use when gas orliquid is ejected from the nozzle the gas or liquid passes through anelectric field created by the electrode.
 10. (canceled)
 11. (canceled)12. (canceled)